1. Field of the Invention
The present invention is directed to the recovery of waste heat from chemical reactions. More particularly, the invention relates to a waste heat boiler with improved mixing of the gas streams exiting the waste heat boiler.
2. Description of the Related Art
Waste heat boilers are most generally used for the generation of steam by waste heat recovered from hot process streams. Typically, those boilers are designed as shell-and-tube exchangers with a plurality of heat exchanging tubes arranged within a cylindrical shell.
Two basic types of shell-and-tube exchangers are employed in the industry, the water-tube type, in which water/steam mixtures flow through the tubes, and the fire-tube type having the heating process stream inside the tubes.
The characteristic components of the boiler are the tubes mounted in tube sheets at a front-end head and a rear-end head within the shell. In the fire-tube boilers steam production is accomplished on the shell side of the tubes by indirect heat exchange of a hot process stream flowing through the boiler tubes. The shell side is through a number of risers and down-comers connected to a steam drum, which may be arranged above or as an integral part of the boiler shell.
The mechanical design and, in particular, dimensioning of the heat exchanging surface in shell-and-tube exchanger type boilers represent certain problems. Fire-tube boiler applications involve high pressures on the shell side or on both sides, and considerable temperature differences between the shell side and the tube side. Particular considerations have to be given to fouling and corrosion characteristics of the process stream.
Boilers handling fouling and/or corrosive process streams must be designed to a higher duty than required when clean in order to allow for satisfying lifetime under serious fouling and/or corroding conditions. The heat exchanging surface of the boiler tubes has further to be adapted to expected corrosion and fouling factors in the stream. To provide for a desired and substantially constant cooling effect during long term operation of the boilers, appropriate heat exchange and temperature control is required.
Conventionally designed boilers are equipped with a by-pass of a relative large diameter tube (relative to the heat exchange tube diameter), which may be internal or external to the boiler shell. The by-pass is usually constructed as an insulated tube provided with a flow control valve. During initial operation of the boilers, part of the hot process stream is by-passed the heat exchanging tubes to limit the heat exchange within the required level.
After a certain time, on stream fouling and/or corrosion of the tubes increase, leading to decreased heat exchange. The amount of by-passed process stream is then reduced, which allows for higher flow of the process stream through the heat exchanging tubes to maintain the required cooling effect. Hence, control of the temperature of the process gas exiting the waste heat boiler is accomplished by varying the flow of the cooled process gas exiting the heat exchanging tubes relative to the flow of the relative hot process gas exiting the by-pass tube.
However, a drawback of the known boilers of the above type is a poor mixing of the cooled process gas and the relative hot process gas exiting the heat exchanging tubes and the by-pass tube respectively of the waste heat boiler. Experience with known waste heat boiler shows that large temperature variations exist in the process gas downstream of the waste heat boiler. This is problematic as for instance the relative hot part of the downstream process gas can lead to corrosion and the temperature variations may entail temperature tensions.
Examples of known art which have sought to solve the problem of poor mixing are disclosed in EP0357907 which discloses a heat exchanger with heat exchanger pipes which run between two chambers and which are flowed through by a fluid and flowed against by another fluid, and with an overflow pipe through which a changeable partial flow of the fluid can be guided to avoid the heat exchange. The overflow pipe is provided with a valve arrangement for the modification of its flow cross-section. This valve arrangement comprises a valve disc, which closes the overflow pipe in one end position of the valve arrangement, and a valve ring which is flowed through by the fluid leaving the overflow pipe and, in the other end position of the valve arrangement, closes an outlet opening for the fluid issuing from the heat exchanger pipes. In order to guarantee a low-loss and intensive mixing of the partial flows of the fluid with greatly reduced space requirement for the mixing section, the outlet opening is formed in a collecting cone which interacts with the valve ring. The valve ring is provided with a conical outlet area which is provided with a great number of penetration openings and the inclination of which to the longitudinal axis of the heat exchanger corresponds approximately to the inclination of the collecting cone.
Another example is disclosed in WO 2012/041344 which describes a waste heat boiler having heat exchange tubes for indirect heat exchange of a relatively hot process gas and a cooling media, and a by-pass tube for by-passing a part of the process gas; a process gas collector collects and mixes a part of the heat exchanged process gas and at least a part of the by-passed process gas before the mix is lead via a control valve to the process gas outlet of the waste heat boiler together with the rest of the heat exchanged process gas.
Further examples of waste heat boilers are described in U.S. Pat. No. 5,452,686A, US2007125317A, U.S. Pat. No. 4,993,367A, GB1303092A, U.S. Pat. No. 1,918,966A and EP0357907A.